Plans have been drawn up to stage an eight-team international 'festival of rugby' in London this autumn, with the Six Nations sides joined by South Africa and Japan.
With continuing uncertainty about the scheduled north versus south November Tests going ahead, officials are scrambling to prepare contingencies.
One of those being considered is a one-off event involving two pools of four leading countries, with the possibility of round-robin fixtures and a final taking place at Twickenham.
The possible Autumn tournament in London would also involve South Africa and Japan.
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At this stage, the preference of all unions is to proceed with the matches which have already been organised, including England's four-match campaign in November against New Zealand, Argentina, Tonga and Australia. However, the quest for fall-back options has created an intriguing alternative proposal.
RFU chief executive Bill Sweeney confirmed on Friday that, as first reported by Sportsmail, one of the fall-back plans involves a 'home and away' Six Nations, starting in November and ending in the usual February-March slot.
But he added: "There's another option of possibly bringing in additional invitational sides. It's an opportunity to be creative and maybe create some type of festival of rugby."
This year's Six Nations was suspended mid-way through due to the coronavirus pandemic
His comments sparked speculation that the Six Nations would be joined by two Barbarians-type sides. Instead, Sportsmail understands that South Africa and Japan could participate, which would be a notable coup for organisers. It would pit the European elite against the world champion Springboks and Jamie Joseph's Brave Blossoms, who were a box-office hit at their home World Cup last year.
Eddie Jones's side are set to face Joseph's men in Oita and Kobe in October if the likely rescheduling of those postponed July Tests is approved.
Rugby's governing bodies are set to meet at a summit on June 15 to discuss the the best way forward for professional rugby, to better align the disparate fixture schedules north and south of the equator.